What is the practical speed limit of space travel?

So; given that there are only a handful of hydrogen molecules present in any square meter of Space, is there a speed at which these interstellar molecules begin to increase drag like in an atmosphere? E.g... like the Sonic barrier requires a specifically shaped wing to overcome..

Would there be increasingly diminished returns within the vicinity of a star? A nebula?

Also, would the compression of spacetime at increasing speeds play a role?

Thanks in advance!